The present invention relates generally to AC-to-AC converters and, more particularly, to AC-to-AC converters for motor drives and method of operation thereof that provides for variable frequency, variable voltage operation of the driven system, as particularly suited for the variable speed operation of electrical machines.
Electrical machines are often controlled by electronic drives (i.e., motor drives), especially to operate them at various speeds. Conventionally, such motor drives rectify the incoming AC voltage, smooth the DC voltage with a capacitor, and then invert it with a six-IGBT inverter. This makes it possible to operate the machine at any speed, since the machine speed is proportional to the frequency generated by the inverter. Many sophisticated controls have been devised for such drives.
Not all machines and applications, however, can make use of such sophistication, and lower cost motor drives would be desirable. Aside from drives, however, the only option on the market, specifically for induction machines, is the “soft starter”, which typically employs silicon controlled rectifiers (SCRs) for reducing the inrush current to the motor but that offer limited control thereover (e.g., while SCRs can be turned on at will, they cannot be turned off in a dynamic and controllable fashion, as SCRs turn off naturally when the current reaches zero). Soft-starters that include IGBTs have been introduced more recently, but up until now such soft-starters have lacked the sophistication and controls necessary for controlling the machine and its load beyond the starting process.
It would therefore be desirable to provide a motor drive and method of operation thereof that provides many of the advantages of a sophisticated drive, but that is less complex and less expensive to manufacture.